CFE: Implement new inference for function literals with voidy return type from context

[eernstg]

Cf. the spec update in dart-lang/language#1092, https://dart-review.googlesource.com/c/sdk/+/154661 changes several language tests to match the new specified behavior, and they currently fail:

language/invalid_returns/async_invalid_return_05_test
language/invalid_returns/async_invalid_return_08_test
language/invalid_returns/async_invalid_return_11_test
language/invalid_returns/async_invalid_return_14_test
language/invalid_returns/async_invalid_return_17_test
language/invalid_returns/async_invalid_return_20_test
language/invalid_returns/async_invalid_return_52_test
language/invalid_returns/async_invalid_return_53_test
language/invalid_returns/async_invalid_return_54_test
language/invalid_returns/async_invalid_return_55_test
language/invalid_returns/async_invalid_return_56_test
language/invalid_returns/async_invalid_return_57_test
language/invalid_returns/async_invalid_return_58_test
language/invalid_returns/async_invalid_return_59_test

Note that #42720 is the corresponding issue for the analyzer, and that work is ongoing already there, just in case this functionality is shared among the analyzer and the CFE.